1. Field of the Invention
The present invention relates to a printing apparatus and print medium conveyance method. Particularly, the present invention relates to a printing apparatus which conveys a print medium such as a print paper sheet and discharges ink from a printhead to print on the print medium, and a print medium conveyance control method.
2. Description of the Related Art
When conveying a print medium such as print paper in a printing apparatus, the following method has conventionally been adopted to prevent the print medium from being diagonally conveyed. More specifically, while a conveyance roller stops or rotates in a direction opposite to the print medium conveyance direction, a feed roller conveys a print medium to make the leading end of the print medium abut against the conveyance roller and to adjust the direction of the print medium. Of such methods, it is well-known to detect the leading end position of a print medium by a print medium position detection sensor arranged immediately before the conveyance roller, then convey the print medium by the feed roller by a distance from the sensor to the conveyance roller, and make the leading end of the print medium abut against the conveyance roller.
However, to increase the throughput, the print medium is generally conveyed to a print start position without performing the control of preventing the diagonal movement of a print medium. In this case, after the print medium position detection sensor arranged immediately before the conveyance roller detects the leading end position of a print medium, the feed roller and conveyance roller convey the print medium by a distance from the sensor to the print start position.
In general, a motor such as a DC motor is used to drive the feed roller and conveyance roller for conveying a print medium. A print medium is generally conveyed by PWM-controlling the motor such as a DC motor. Japanese Patent Laid-Open No. 2002-347296 discloses an arrangement in which when the count at which the duty of a pulse signal supplied to a DC motor becomes a maximum value reaches a predetermined value in PWM-controlling the DC motor, the duty is reset to 0 (zero). When the DC motor is locked, power supply to the DC motor stops to prevent heat-up of the DC motor.
When preventing the diagonal movement of a print medium by the conventional PWM control, abutting of the leading end of a print medium is determined on the basis of whether or not a PWM value serving as the duty of a pulse signal supplied to a motor has exceeded a threshold. However, if feed conditions such as the type of print medium, the arrangement of the conveyance mechanism, and motor performance change, the timing when the PWM value exceeds the threshold also changes. It may be impossible to accurately prevent the diagonal movement of a print medium.
FIG. 10 is a graph for explaining a control method of preventing the diagonal movement of a print medium using the conventional PWM control.
When the PWM value exceeds a predetermined PWM threshold, it is controlled to reset the PWM value to 0 (zero) and stop the conveyance of a print medium. In this case, an abutting detection timing error AX (FIG. 10) may occur depending on a PWM value obtained before the print medium abuts against the conveyance roller. For this reason, the print medium may not be able to satisfactorily abut against the conveyance roller, failing to prevent the diagonal movement of the print medium. To the contrary, the print medium may excessively abut against the conveyance roller and be wrinkled.
The conventional printing apparatus assumes that the feed roller always has a constant frictional force against a print medium and conveys a print medium by this frictional force. However, as the number of fed print media increases, the feed roller may wear or paper dust of a print medium may be attached to the feed roller. If the feed roller wears or paper dust of a print medium is attached to the feed roller, the feed roller and print medium slip during the feed operation. The amount of rotation of the feed roller and the actually moving amount of the print medium may become different from each other. As a result, the feed roller may not be able to accurately convey a print medium. Particularly, when increasing the throughput without performing the control of preventing the diagonal movement of a print medium, the print medium may not be able to be accurately conveyed to the print start position.